1. Field of the Invention
This invention relates to an ultrasonic motor utilizing ultrasonic vibration and an ultrasonic actuator.
As an ultrasonic motor of this kind, there is known a "heteromorphic degeneration longitude L1-bend B4 mode planar motor" described, for example, in "the Lecture Papers in the 5th Dynamics Symposium Related to Electromagnetic Force".
FIGS. 4A to 4C of the accompanying drawings show the structure of the ultrasonic motor described in the above-mentioned publication. FIG. 4A is a view of the ultrasonic motor as it is seen from just above it, FIG. 4B is a cross-sectional view of the ultrasonic motor taken in the direction of arrow P, and FIG. 4C is a cross-sectional view of the ultrasonic motor taken in the direction of arrow Q.
In FIGS. 4A to 4C, the reference numeral 1 designates a resilient member having piezoelectric elements 11 and 12 adhesively secured to the upper surface thereof, and electrodes, not shown, are adhesively secured to the upper surfaces of the piezoelectric elements 11 and 12. Also, projected portions 13 and 14 are formed on the lower surface of the resilient member 1, and vibration created in the resilient member 1 is taken out by these projected portions 13 and 14. These projected portions 13 and 14 will hereinafter be called the drive force taking-out portions. The piezoelectric elements 11 and 12 are polarized in the same direction, and high frequency voltages differing in phase by 90 degrees (.pi./2) from each other are applied to the respective piezoelectric elements 11 and 12 through the electrodes.
2. Related Background Art
FIG. 7 of the accompanying drawings shows the relation between the frequency of the high frequency voltages applied to the piezoelectric elements 11 and 12 of the ultrasonic motor of FIGS. 4A to 4C and the amplitude of the vibration created in the resilient member 1.
As shown in FIG. 7, as the frequency of the high frequency voltages is gradually dropped from a maximum frequency f.sub.max, the amplitude of the vibration becomes gradually greater. When the frequency of the high frequency voltages becomes lower than a frequency f.sub.b for which the amplitude of the vibration becomes maximum, the amplitude of the vibration suddenly decreases and the ultrasonic motor becomes stopped. The frequency for which the shown amplitude of the vibration becomes greatest is generally called the resonance frequency, and when the ultrasonic motor is driven at this frequency, the ultrasonic motor can be driven most efficiently.
On the other hand, as the frequency of the high frequency voltages is gradually increased from a minimum frequency f.sub.min, the amplitude of the vibration suddenly increases at a point of time whereat the frequency exceeds a frequency fa which is a frequency higher than the frequency fb, and the ultrasonic motor begins to be driven and thereafter, the amplitude of the vibration decreases gradually.
In the ultrasonic motor shown in FIGS. 4A to 4C, the frequency of the high frequency voltages applied to the piezoelectric elements 11 and 12 can be varied to thereby control the speed of the ultrasonic motor. However, when an attempt is made to drive the ultrasonic motor, for example, at a frequency between the shown frequencies fb and fa, if the frequency is gradually dropped from the maximum frequency f.sub.max side, the ultrasonic motor can be driven within the above-mentioned frequency range without any problem, whereas if the frequency is gradually increased from the minimum frequency f.sub.min side, the ultrasonic motor remains stopped within the above-mentioned frequency range as shown in FIG. 7. Accordingly, when for example, in an attempt to drive the ultrasonic motor at the resonance frequency, the frequency of the high frequency voltages are variously varied to effect the retrieval of the resonance frequency, there is the possibility that in some cases, the frequency may be dropped too much and the ultrasonic motor may become unable to be started.
Further, FIG. 18 of the accompanying drawings is a perspective view showing an example of an ultrasonic actuator of the longitudinal and torsional vibration type according to the prior art.
In an ultrasonic actuator of this kind, a stator 201 (see FIG. 19) is such that a piezoelectric element 204 for torsional vibration is interposed between two vibrators 202 and 203 of the cylinder type and a piezoelectric element 205 (see FIG. 19) for longitudinal vibration is disposed on the upper side of the vibrator 203. The piezoelectric element 204 for torsional vibration is polarized circumferentially thereof, and the piezoelectric element 205 for longitudinal vibration is polarized in the direction of the thickness thereof. Further, a rotor 206 is disposed on the upper side of the piezoelectric element 205 for longitudinal vibration.
The vibrators 202, 203 and piezoelectric elements 204, 205 constituting the stator 201 are fixed to a shaft 207 (threadably engaged with the threaded portion of the shaft 207), and the rotor 206 is rotatably provided on the shaft 207 through a ball bearing 208. The tip end of the shaft 207 is threadably engaged by a nut 210 through a spring 209 to thereby bring the rotor 206 into pressure contact with the stator 201.
The piezoelectric element 204 for torsional vibration and the piezoelectric element 205 for longitudinal vibration are driven by a voltage of the same frequency oscillated by an oscillator 211 which is phase-controlled by a phase device 212.
The piezoelectric element 204 for torsional vibration gives mechanical displacement for the rotor 206 to rotate, and the piezoelectric element 205 for longitudinal vibration performs the function of synchronizing a frictional force working between the stator 201 and the rotor 206 with the period of the torsional vibration by the piezoelectric element 204 and periodically fluctuating it, thereby converting vibration into motion in one direction.
FIG. 19 of the accompanying drawings is a developed perspective view showing the stator of the ultrasonic actuator according to the prior art.
The piezoelectric element 204 for torsional vibration need be polarized circumferentially thereof and therefore. As shown in FIG. 18, a piezoelectric material has once been divided into six to eight sector-shaped small pieces, and each small piece has been polarized, whereafter the small pieces have been again combined into an annular shape. The reference character 204a designates an electrode.
However, in the aforedescribed prior-art ultrasonic actuator, it has been difficult to yield shape accuracy when the piezoelectric elements for torsional vibration are combined into an annular shape.
On the other hand, the areas of the piezoelectric elements for longitudinal vibration and for torsional vibration have been substantially equal to or smaller than the cross-sectional area of the stator. Also, in order to pass a shaft through the piezoelectric elements, it has been necessary to form a hole in the central portions of the piezoelectric elements. Therefore, the areas of the piezoelectric elements have become smaller and it has been difficult to obtain high torque and high-speed rotation of the motor.
In order to solve such problems, applicant has already proposed an ultrasonic actuator of the longitudinal and torsional vibration type which can be driven by high torque and high-speed rotation and moreover is simple in structure and simple to manufacture (Japanese Patent Application No. 6-180279).
The stator of this ultrasonic actuator of the longitudinal and torsional vibration type is of a construction which comprises a thick resilient member divided into semicircular tubular shapes, and electro-mechanical conversion elements for torsional vibration and longitudinal vibration joined to the divided surfaces of the resilient member (see FIGS. 12A and 12B of the accompanying drawings). The rotor of the ultrasonic actuator is disposed on the end surface (driving surface) of the stator for rotation about a shaft and is brought into pressure contact with the driving surface. When each electromechanical conversion element is excited by the application of a driving signal thereto, torsional vibration and longitudinal vibration are created in the resilient member. When the resonance frequencies of the longitudinal vibration and torsional vibration substantially coincide with each other, longitudinal vibration and torsional vibration are created at a time (degeneration), elliptical motion is created in the driving surface and a drive force is generated, whereby the rotor is rotated.
In any of the aforedescribed ultrasonic actuators of the axial and torsional vibration type, the axial vibration serves as a clutch for the stator and rotor and the torsional vibration serves to impact a rotational force to the rotor and therefore, when the rotor is to be driven, it is driven by the vicinity of the resonance frequency of the torsional vibration.
FIG. 17 of the accompanying drawings is a waveform graph for illustrating a case where the resonance frequencies of the longitudinal vibration and torsional vibration of the ultrasonic actuator of the longitudinal and torsional vibration type deviate from each other.
In this ultrasonic actuator, there is a case where when the resonance frequency .omega..sub.0L of the longitudinal vibration is greater than the resonance frequency .omega..sub.0T of the torsional vibration, the frequency creating the torsional vibration and driving the actuator becomes smaller than the resonance frequency of the longitudinal vibration.
This ultrasonic actuator can be stably driven at a frequency higher than the resonance frequency, but the stable driving thereof is difficult at a frequency lower than the resonance frequency. Accordingly, if the frequency becomes smaller than the resonance frequency .omega..sub.0L of the longitudinal vibration when the ultrasonic actuator is being driven by the driving frequency range of the torsional vibration, the longitudinal vibration may become unable to serve as a clutch for the stator and rotor. This has led to the problem that rotational motion becomes unstable and along therewith, the drive force and driving efficiency are reduced.
On the other hand, such an ultrasonic actuator has generally been designed such that the resonance frequencies .omega..sub.0L and .omega..sub.0T of the longitudinal vibration and torsional vibration are made coincident with each other.
However, the actually manufactured resilient member has been such that the resonance frequencies of the longitudinal vibration and torsional vibration deviate from each other and the resonance frequency of the longitudinal vibration becomes higher or the resonance frequency of the torsional vibration becomes higher.
Accordingly, the prior-art ultrasonic actuator has suffered from the problem that depending on the manufacture thereof, there is a case where stable driving is obtained and there is a case where stable driving is not obtained, and this gives birth to an individual difference in performance.